Recently, various devices have been under active study and development, particularly those based on electroluminescence (EL) from organic materials.
The use of phosphorescent materials has been a major breakthrough in boosting electroluminescence efficiency since they allow simultaneous harvesting of both singlet and triplet excitons.
Unfortunately, the emission lifetimes of these phosphorescent complexes are relatively long, leading to undesired triplet-triplet annihilation during the operation of a device. To overcome this problem, phosphorescent emitters are doped into organic host materials.
Selecting a suitable host material for phosphorescent dopants remains one of the critical issues in phosphorescence-based OLEDs.
An ideal host material would meet the following intrinsic requirements: a triplet energy gap (Et) larger than that of the triplet dopant to prevent reverse energy transfer from the guest back to the host, good carrier transporting properties to balance the charge flux and reduce the driving voltage, thermal and morphological stability to extend the device operational lifetime.
Well-known host materials for guest-host systems include hole-transporting 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) and electron-transporting aluminum 8-hydroxyquinoline (Alq3), which have been used in OLEDs. Those host materials have suitable properties for green and red emitters.
In contrast, highly efficient blue-light emitting phosphorescent devices remain rare, mainly because of the lack of suitable host materials possessing both charge transporting characteristics and high triplet energy.
Several host materials for better phosphorescent emission have been reported. Due to their charge conducting ability, photophysical and redox properties, sufficiently large triplet energies and carrier-transport properties, carbazole-based compounds have been actively studied.
For example, U.S. Patent Application Publication No. US 2003/205696 discloses guest-host emissive systems suitable for use with organic light emitting devices in which the host material comprises a compound having a carbazole core with an electron-donating species bonded to nitrogen, aromatic amine groups or carbazole groups bonded to one or more of the carbon atoms, a large band gap potential, and high-energy triplet excited states. Such materials permit short-wavelength phosphorescent emission by an associated guest material, and the combination of said materials with emissive phosphorescent organometallic compounds such as iridium complexes is useful in the fabrication of organic light emitting devices.
U.S. Patent Application Publication No. US 2005/0031899 discloses carbazole derivatives used as an organic semiconductor element, a light emitting element and an electronic device by employing the carbazole derivative. N-benzyl-3,6-di-(N-carbazolyl)carbazole is used as starting material for the preparation of 3,6-di-(N-carbazolyl)carbazole in synthesis example 4.
U.S. Patent Application Publication No. US 2009/080799 discloses norbornene-monomer, poly(norbornene) homopolymer, and poly(norbornene) copolymer compounds containing a functionalized carbazole side chain used as hole transport and/or electron blocking materials and as organic host materials for an organic luminescence layer and an OLED device.
Further, Lengvinaite et al., “Carbazole-based aromatic amines having oxetanyl groups as materials for hole transporting layers,” Synthetic Metals, 157: 529-533 (2007), discloses several oxetane-functionalized carbazole-based aromatic amines.
Tsai et al., “3-(9-carbazolyl)carbazoles and 3,6-di(9-carbazolyl)carbazoles as effective host materials for efficient blue organic electrophosphorescence,” Adv. Mater., 19: 862-866 (2007), and Tsai et al.,
“P-152: Efficient blue phosphorescent OLEDs employing novel oligocarbazoles as high-triplet-energy host materials,” SID 07 DIGEST, 38(Bk. 1): 772-775 (2007), disclose a strong dependence of the linking topology on the electronic coupling between monomeric carbazole units for directly linked oligocarbazoles.
Knights et al., “A rapid route to carbazole containing dendrons and phosphorescent dendrimers”, J. Mater. Chem. 2008, 18, 2121-2130, discloses N-benzyl-3,6-di(N-Carbazolyl)carbazole, wherein the carbazole substitutents in 3 and 6 position are substituted themselves by fluorene substitutents.
Radecki et al., “Oligocarbazoles as ligands for lead-selective liquid membrane electrodes”, Analyt. Sci., November 2004, Vol. 20, 1599-1603 discloses N-benzyl-3,6-di(N-(3′,6′-di-tert.butyl)carbazolyl)carbazole as ionophore in liquid membrane electrodes for lead determination in water samples.
Hameurlaine et al., “Synthesis of soluble oligocarbazole derivatives, Tetrahedron Letters, Vol. 44 No. 5, 2003, 957, also discloses N-benzyl-3,6-di(N-(3′,6′-di-tert.butyl)carbazolyl)carbazole as a building block in the synthesis of trimeric and heptameric carbazoles with good solubilities in organic solvents.
It is described in the above literatures that 3(6), 9′-linked oligocarbazoles investigated exhibit fairly high thermal stability. By adjusting the thickness of the hole-transport layer in the OLED device comprising the alkyl chain substituted triscarbazole (e.g., 2-ethylhexyl triscarbazole), lower operating voltages and higher power efficiencies were observed. However, none of the above-disclosed materials meets all the requirements necessary for OLED application, e.g., suitable energy level, charge transport ability, processibility from a solution with uniform film formation, ability to form an amorphous phase, ability for good dopant dispersion, morphological stability (high Tg), and thermal and electrochemical stabilities under operational conditions of the device. For example, substitution of branched alkyl chain on triscarbazoles (e.g., 2-ethylhexyl triscarbazole) increases the solubility in organic solvent but decreases the glass transition temperature (Tg) to thereby make the material difficult to sublimate while lowering the stability of glassy film morphology in the device leading to degraded lifetime. Tsai et al. also mention in the above literatures that for the same oligomer length, substitution by rigid and bulkier groups gives higher Tg values than alkyl substitution, and high Tg values for these new host materials are also expected to benefit the stability of the devices.
Thus, there has been a need to develop new host materials, which are capable of satisfying all of the requirements indicated above.